One of Europe’s oldest human rights bodies is being urged to set up a far-reaching anti-corruption investigation next week, amid fresh allegations of vote rigging that have put its credibility on the line.

Two people with high-level experience of the Council of Europe’s parliamentary assembly (Pace) have told the they believe its members have been offered bribes for votes by Azerbaijan. The 324-member body is made up of delegates from national parliaments who meet four times a year in Strasbourg.

Arif Mammadov, a former Azerbaijani diplomat turned dissident, alleged that a member of the oil-rich country’s delegation at the Council of Europe had €30m (£25m) to spend on lobbying its institutions, including the Council of Europeassembly.

“Everyone” in the Azerbaijani delegation had heard of this number, although “it was never written down”, he told the Guardian. “It was said this money was to bribe members of the delegations and Pace generally.”

Tobias Billström, a Swedish delegate to the assembly and former justice minister, said “very credible members” had told him they had been offered bribes to vote in a certain way. He is one of 64 parliamentarians to have signed a resolution calling for an independent investigation into “serious and credible allegations of grave misconduct” centred on an Azerbaijani vote.

Allegations of “caviar diplomacy” have swirled around the Council of Europe’s parliamentary assembly for years, with Azerbaijan accused of offering cash and luxury gifts in exchange for favourable votes.

The claims were first laid out in a 2012 report by the European Stability Initiative thinktank, but have gathered momentum since Italian prosecutors began investigating a former chair of the centre-right group, Italian deputy Luca Volontè.

Volontè is accused of accepting €2.39m in bribes from Azerbaijan in exchange for supporting its government in the Council of Europe. He faces a trial for money laundering, and Milan’s public prosecutor is appealing a decision to drop a corruption charge against him. He has always denied any wrongdoing.

Although one fifth of MPs at the Council of Europe called for an urgent inquiry in January, assembly leaders failed to take a decision at their last meeting in March.

Pressure is building on the assembly president, Pedro Agramunt, to ensure that a robust investigation is set up when it meets later this month.

Agramunt, a Spanish centre-right politician, is already facing criticism for meeting the Syrian president, Bashar al-Asssad, on a Syria visit organised by the Russian government. He was accompanied by Leonid Slutsky, the head of the Russian parliament’s foreign affairs committee, as well as MPs from Belgium, Italy and Serbia, according to Russian media.

Members of the assembly believe the trip, which came two weeks before chemical attacks near Idlib, could tarnish the credibility of the Council of Europe as a human rights defender. The Socialist group has declared itself “extremely concerned and worried” and the head of the French delegation has made an official complaint.

Critics say Azerbaijan uses the assembly to add a veneer of legitimacy to the authoritarian rule of its president, Ilham Aliyev, who has ruled the country since 2003.

One case concerns the decision of assembly members in 2013 to vote down a critical report on political prisoners in Azerbaijan by German social democrat Christoph Strässer. The Strässer report concluded that Azerbaijan’s judicial system was used to silence or intimidate critics of the Aliyev regime and was rejected by 125 votes to 79 with 20 abstentions.

Volontè is alleged to have played a key role in orchestrating the defeat with payments to him channelled through a company with a connection to Azerbaijan’s ruling family, according to a recent report by the investigative journalist Khadija Ismayilova, published by the Organized Crime and Corruption Reporting Project.

Strӓsser said he knew Azerbaijan was giving out money but had no proof votes were bought. He said he wanted to see an investigation “as a point of credibility”.

“If there is any suspicion that there could be corruption within these structures and this was ignored, I think it might be the end of the Council of Europe.”